Fire protection and suppressant systems which operate by detecting the presence of smoke and other airborne pollutants are well known. Upon a threshold level of smoke being detected, an alarm may be activated and operation of a fire suppressant system may be initiated. While the fire itself will cause damage, considerable damage can also be caused by operation of the fire suppression system and subsequent removal of the suppressant can be quite hazardous. Many traditional suppressants, such as halon, are also ozone depleting whereby this use is environmentally undesirable. A detection system which is sufficiently sensitive to detect an abnormal condition prior to the onset of a fire is very advantageous as it enables action to be taken at a very early stage before the onset of actual fire conditions. For example, when most substances are heated, even before heating occurs to a point at which a fire commences, emissions will be generated and if these can be detected by a very sensitive system, a warning provided at that very early stage may allow the problem to be detected and rectified, or the equipment turned off, before the fire actually starts.
It is also desirable for the detection system to have a wide dynamic range of operation whereby it is effective not only at low levels of smoke and other airborne pollutants as may be generated prior to the onset of actual fire conditions as discussed above, but also is able to detect a range of higher threshold levels of smoke and other pollutants. High levels of smoke will indicate a greater likelihood of there being a fire and the higher thresholds can trigger alarms to shut down air conditioning, close fire doors, call a fire fighting service, and eventually trigger a suppression system if the smoke level becomes sufficiently high.
It is known for detection systems to incorporate a sampling pipe network consisting of one or more sampling pipes with sampling holes installed at positions where smoke or pre-fire emissions can be collected. Air is drawn in through the sampling holes and along the pipe by means of an aspirator or fan and is directed through a detector at a remote location.
Although there are a number of different types of smoke detectors which can be used as the detector in a system as outlined above, one particularly suitable form of detector for use in such a system is an optical scatter detector which is able to provide good sensitivity at reasonable cost. Optical scatter detectors operate on the principle that smoke particles or other airborne pollutants of small size when introduced into a detection chamber having a high intensity light beam will cause light scatter. The scattered light is sensed by a scattered light detector. The greater the amount of smoke particles within the sample introduced into the detector chamber the greater will be the amount of light scatter. The scatter detector detects the amount of scattered light and hence is able to provide an output signal indicative of the amount of smoke particles or other pollutant particles within the sample flow.